Essential tremor (ET) is one of the most common movement disorders, estimated to affect 1 - 12 million people in the US. It is characterized by a postural and kinetic tremor in the upper limbs, making activities of daily living (eating, clothing writing, etc.) difficult or impossible. Medication and surgical interventions have significantly reduced patient suffering, but medication only reduces tremor by 50%, and only in 50% of patients, and surgical procedures are generally reserved for severe drug-resistant tremor and have mild to severe side effects, leaving many ET patients without effective treatment options. Surprisingly, assistive and rehabilitative devices are virtually unexplored in this disorder despit success in other neurological disorders involving movement. One might envision, for example, a wearable upper limb device (e.g. an orthosis) with inertia, damping and stiffness specifically designed to suppress (mechanically low-pass filter) tremor in ET patients. However, a significant obstacle to developing effective assistive and rehabilitative devices is that the characteristics o ET are not known throughout the upper limb. Because most studies have only investigated the tremor in a single degree of freedom (most often wrist flexion-extension), we do not know where in the upper limb the tremor manifests most severely, where it originates, and how it propagates, greatly limiting our ability to effectively treat ET with assistive or rehabilitative approaches. The purpose of this work is to characterize tremor (in terms of severity and frequency) in all major DOF of the upper limb in 40 patients with ET (Aim 1), and to apply heuristic and optimization models to the data to establish the mechanical origin of the tremor (i.e. which muscles cause the tremor), and how the tremor propagates from its mechanical origin to other joints of the upper limb (Aim 2). This work will enable the future development of an optimal tremor suppression strategy. Because voluntary movements and ET generally occupy different frequency bands, it is theoretically possible to add mechanical impedance (stiffness, damping, and inertia) to passively low-pass filter the tremor while leaving the voluntary movements relatively unperturbed. However, optimal tremor suppression requires knowledge of where to suppress tremor and how to suppress tremor in order to produce the maximum attenuation of the tremor and the minimum attenuation of voluntary movements. Building on the work proposed here, we will apply methods from the field of passive vibration control to determine the optimal strategy for minimizing tremor at the hand (the most common site of interaction with the environment).